Low-density thermoplastic foams, particularly those having a density of less than about 150 kg/m.sup.3, are generally produced with physical blowing agents. Physical blowing agents are chemical compounds which can be incorporated into the thermoplastic melt while within the extruders at high pressures, typically 10-20 MPa, and which can be contained by the polymer structure when the cooled thermoplastic/blowing agent mixture is rapidly reduced to ambient pressure. Polyolefinic foams, in particular, uncrosslinked polyethylene foam and uncrosslinked polypropylene foam, were manufactured for many years with halogenated hydrocarbons, primarily chlorofluorocarbons (CFC's) as physical blowing agents.
During the 1980's the worldwide scientific community presented sufficient evidence to link CFC's with atmospheric ozone depletion and sought for world governments to regulate CFC's. Ozone levels in the stratosphere have been shown to be significant in protecting life on the planet from the damaging effects of ultraviolet radiation from the sun. Additionally, hydrochlorofluorocarbons (HCFC's), another class of chemical compounds were also included in the regulations but on a more lengthy schedule. It was established that the higher atomic weight halogen compounds other than fluorine that were responsible for this effect.
As a result of the regulations, it became necessary to find materials other than halogenated compounds which could function as physical blowing agents. Many different approaches were taken, but generally in the direction of the use of hydrocarbons containing from two to five carbons or blends thereof. Although these short-chained hydrocarbons may function as physical blowing agents to produce foams with satisfactory physical properties, these physical blowing agents are highly flammable. U.S. Pat. No. 4,217,319 (Komori) discusses a process for producing polyolefin foams with various volatile organic compounds as the physical blowing agent. U.S. Pat. No. 5,290,822 (Rogers et al) and U.S. Pat. No. 5,225,451 (Rogers et al) discuss processes for ultra-low density foam using polymer blends. U.S. Pat. No. 5,059,376 (Pontiff) discusses a process to remove residual hydrocarbons in the attempt to produce a non-flammable polyethylene foam.
Carbon dioxide, alone or in combination with various other gases, has been used as a blowing agent to produce polyethylene foam. U.S. Pat. No. 5,034,171 (Kiczek et al) discusses an extrusion process for producing a "microcellular" thermoplastic foam and cites polyethylene as possible polymer and carbon dioxide as a possible inert gas foaming agent. U.S. Pat. No. 5,462,974(Lee) discusses a foamable polyethylene resin composition having a blowing agent comprised of a blend of about 50% by weight of carbon dioxide and about 50% by weight of either normal butane, isobutane, propane, ethane, or a mixture of any combination of these hydrocarbons, in mixture with the resin in a ratio of about one part blowing agent to about 10 parts resin while using a zinc oxide nucleation agent and a glycerol monostearate aging modifier. U.S. Pat. No. 5,416,129 (Chaudhary et al.) and U.S. Pat. No. 5,554,661 (Chaudhary et al.) discuss a process and composition for preparing uncrosslinked ethylenic polymer foam using a polymer of a defined melt tension with either argon, carbon dioxide, or blends thereof. These patents generally state that argon and carbon dioxide, alone or in blends, can function as a physical blowing agent with ethylenic materials having a specific melt tension.
U.S. Pat. No. 5,116,881 (Park et al.), U.S. Pat. No. 5,149,579 (Park et al.), and U.S. Pat. No. 5,180,751 (Park et al.) discuss a process and a composition for producing thermoformable polypropylene foam sheets from high melt strength polypropylene and refer to carbon dioxide as a potential blowing agent. The patent discusses foam sheets in the range of densities above 2.5 lb/ft.sup.3 (42 kg/m.sup.3) with sheet thicknesses below 0.200 inch (5.0 mm) which are defined by certain physical properties. U.S. Pat. No. 5,567,742 (Park) discusses a dimensionally stable polypropylene foam with an inorganic blowing agent. While applicable to both open and closed cell foams, the discussed process with inorganic blowing agents is limited to combinations of cell sizes and densities which result in cell wall thicknesses within a specified range.
When used as physical blowing agents in traditional low density polyolefinic foam processes, carbon dioxide either alone or in combination with other gases including water generally produce non-descript masses of polymeric material or otherwise poor quality thermoplastic foams which collapse. This lack of dimensional stability occurs because of the limited solubility for these compounds within the molten thermoplastic extrudate which results in an uncontrollable high level of open cells in the foam structure as the thermoplastic/blowing agent combination exits the die. Additionally, even if the resultant thermoplastic materials which are made with carbon dioxide as the primary blowing agent have a visible foam structure, most of such foams tend to collapse quickly in time and become useless for most practical applications in less than 24 hours after manufacture. Accordingly, in order to use carbon dioxide as the primary blowing agent, it is necessary to reduce the pre-foaming pressure required to make a foam at the die and subsequently either eliminate or significantly reduce the rate of collapse normally exhibited by polyolefinic foam made with carbon dioxide, water or blends thereof.